This invention relates to the domain of optical instrumentation and in particular the domain of optical spectrometers working with a large free spectral range combined with a high spectral resolution.
It is particularly but not exclusively applicable to optical spectrometers for installation in space exploration vessels and in a rover for planetary exploration.
High spectral resolution spectrometers usually comprise an entry slit for the electromagnetic radiation beam to be analyzed, a collimator to conform the beam that penetrated through the slit so as to make the rays of the beam parallel, a dispersion device such as a prism and/or grating to decompose the beam into parallel rays depending on its different wavelength components in wavelength dependent directions, and an imaging device to focus all the parallel rays on a wavelength dependent position onto a detector.
A specific class of spectrometer with a combination of high spectral resolution and a large free spectral range is the Echelle spectrometer. In such a spectrometer, the radiation beam applied to the entrance slit is collimated by a first set of lenses, then the collimated beam is dispersed by a first dispersion device in a first direction, followed by a second dispersion device which disperse each component output from the first dispersion device once again in direction perpendicular to the first direction, and an imaging device comprising a set of lenses to focus the beam output from the second dispersion device onto a sensitive surface of a sensor on which an image of the entrance slit is formed for each spectral component of the radiation transmitted by the entrance slit. Usually, the first dispersion device is a prism and the second dispersion device is a diffraction grating decomposing the beam by reflection. An example of a spectrometer of this type is described in U.S. Pat. No. 5,859,702.
In order to obtain a high spectral resolution with a large free spectral range, the diffraction grating is used with a high diffraction order, typically of the order of 100, while the prism is used with a low diffraction order to avoid overlapping of the different diffraction orders of the grating.
Particularly due to the presence of a prism, this type of spectrometer is large and heavy which makes it unusable for space planetary missions. This disadvantage is particularly bothersome because the prism must be relatively large so as to be able to process a large free spectral range and to prevent overlapping of the diffraction orders.
Furthermore, the use of diffraction gratings at high diffraction orders is required by the high spectral resolution.
The use of a prism as a first dispersion device makes it necessary to use a collimator.
Besides, linearly variable optical filters have been developed composed of a transparent substrate, one face of which is covered by a multi-layer structure deposited under a vacuum, with a small thickness that varies linearly in one direction and is constant in a perpendicular direction. When a beam is applied on the filter, the wavelength band that passes through the filter depends on the thickness of the multilayer structure and thus on the location at which the beam is applied.
This type of filter is used in a color measurement spectrometer described in patent U.S. Pat. No. 6,057,925 to select a wavelength band within the spectrum to be analyzed and to image it onto the detector. The spectral resolution of this spectrometer is limited to the spectral transmission of the linearly variable optical filter, which is in the order of 10 nm Full-Width-Half-Maximum (FWHM) in the visible wavelength domain. This low spectral resolution is insufficient to analyze atomic emission lines, which require a spectral resolution equal or better than 0.1 nm.